Interface Kit tests: add test for BTextControl.

* Only tests very basic functionality, and size of the class, which must
not change for BeOS compatibility.
* Also add "manual" test for disabled and invalid controls (just check
it looks right)
Introduce "invalid" text controls.

* MarkAsInvalid is used to enable or disable the mark
* The B_INVALID BControlLook flag is used
* invalid BTextControls are drawn with a red border.
* You ar encouraged to let the user know more precisely what's wrong, by
showing an helpful error message next to the control or in a tooltip.